Aromatic hydrocarbons, such as benzene, toluene, xylene, etc., are useful as fuels, solvents, and as feeds for various chemical processes. Of the xylenes, paraxylene is particularly useful for manufacturing phthalic acids such as terephthalic acid, which is an intermediate in the manufacture of synthetic fibers such as polyester fibers. Xylenes can be produced from naphtha, e.g., by catalytic reforming, with the reformate product containing a mixture of xylene isomers and ethylbenzene. Separating paraxylene from the mixture generally requires stringent separations, e.g., separations utilizing superfractionation and multistage refrigeration steps. Such separations are characterized by complexity, high energy-usage, and high cost.
Chromatographic separation is an alternative to more stringent separations, such as superfractionation, for removing paraxylene from a mixture of aromatic C8 isomers. Chromatographic separation involves simulating a moving bed of selective adsorbent. Examples of commercial processes in which paraxylene is separated from aromatic C8 isomers by simulated moving bed separation (“SMB”) include PAREX, available from UOP, ELUXYL, available from Axens, and AROMAX, available from Toray.
Traditionally, there are two outlet streams in an SMB operation, raffinate and extract, for the recovery of the products. The raffinate stream may be isomerized and recycled to the paraxylene separation step to produce more paraxylene (commonly referred to as a xylene loop). Historically, xylene isomerization has been accomplished in the vapor phase. However, liquid isomerization units have recently found increasing use in paraxylene separation systems. One drawback of liquid phase isomerization is that liquid phase isomerization converts little or none of the ethylbenzene in the paraxylene-depleted stream, and as a result, the amount of ethylbenzene in the xylenes loop can build up to very high levels.
US 2015/0266794A discloses a xylene loop in which a paraxylene-containing extract and an ethylbenzene-containing raffinate are separated from a first mixture in a first separation stage to form a paraxylene-depleted raffinate. The paraxylene-depleted raffinate and the ethylbenzene-containing raffinate are withdrawn in such a way that the majority of the ethylbenzene (EB) is withdrawn in the ethylbenzene-containing raffinate. This uneven split in EB enables the process to primarily use liquid phase isomerization (LPI) for the paraxylene-depleted raffinate. The ethylbenzene-containing portion can be removed from the process to recover ethylbenzene or can be subjected to a vapor phase isomerization.
However, prior to isomerization, it generally is required to remove desorbent from the C8 aromatics as the desorbent may crack during the isomerization, which can result into a downgrading of the desorbent, and is not economical. Typically, a SMB system using PDEB (para-diethylbenzene) as a desorbent (“heavy” SMB unit) will have an extract tower to separate desorbent from the extract stream which comprises paraxylene and desorbent, finishing tower(s) to remove any other lighter hydrocarbons, such as toluene remaining from the feed, from the paraxylene, and raffinate tower(s) to separate desorbent from the raffinate stream which comprises orthoxylene, metaxylene, and ethylbenzene. A SMB system using toluene as a desorbent (“light” SMB unit) only needs the extract and raffinate towers, since the extract tower separates out both the toluene in the desorbent stream as well as trace toluene in the xylene feed. Where two raffinates are withdrawn from a SMB system as described in US 2015/0266794A, two raffinate towers will be required for recovery of desorbent. This increases the capital investment, especially for a retrofit applications of a SMB system.
Thus, there is a need for a process for producing paraxylene that achieves a high paraxylene separation efficiency, avoids the undesired accumulation of ethylbenzene in the xylene loop, and reduces the need for distillation towers, thereby lowering the costs, while avoiding degradation and loss of the desorbent.